Recent high gasoline prices and increased consumer demand have resulted in numerous efforts to reduce our dependence on petroleum as a source of energy. Ethanol, and the blending of ethanol with gasoline used to fuel our automobiles, holds substantial promise at reducing our consumption of petroleum. In fact, ethanol blending is mandated by the federal and state governments in many cases.
Unfortunately, the blending of ethanol into our petroleum supply has created its own set of problems, particularly for air quality control. The problem is that there are multiple suppliers of ethanol and gasoline in the petroleum distribution system, and that the ethanol and gasoline from different suppliers can react differently, to produce different physical properties for the blend, particularly in terms of volatility, a key component of any air quality control program.
The problem is magnified when other components of our petroleum supply, such as butane, are factored in. Butane is often added to the gasoline supply to improve its combustibility and to decrease its overall cost, but butane blending is only permissible under certain conditions, and at certain times of year, based on air quality specifications. The fact that ethanol will be added to the gasoline after butane is blended only complicates the matter, because butane must be blended based on an interaction between gasoline and ethanol that cannot be predicted in advance.
Furthermore, ethanol, unlike gasoline, is not suitable for transportation through pipelines because of its high affinity for water, and is most often blended with gasoline after it has been transported and blended with butane. In view of this imprecision, gasoline suppliers are unable to optimize the amount of butane that they can blend with gasoline. Thus, a need exists for the ability to blend butane with gasoline that is to be mixed with ethanol in an amount that does not cause the final blend to exceed predetermined volatility limits.
There are three principal methods for assessing the volatility of gasoline: (1) measuring the vapor to liquid ratio, (2) measuring the vapor pressure, and (3) measuring the distillation temperature. The Reid method is a standard test for measuring the vapor pressure of petroleum products. Reid vapor pressure (RVP) is related to true vapor pressure, but is a more accurate assessment for petroleum products because it considers sample vaporization as well as the presence of water vapor and air in the measuring chamber. The distillation temperature is another important standard for measuring the volatility of petroleum products. When blending gasoline with volatility modifying agents, the distillation temperature (TD) often cannot fall below a prescribed value. TD refers to the temperature at which a given percentage of gasoline volatilizes under atmospheric conditions, and is typically measured in a distillation unit. For example, the gasoline can be tested for T(50), which represents the temperature at which 50% of the gasoline volatilizes, or it can be measured at T(10), T(90), or some other temperature value.
Several methods have been attempted to improve the precision of blending and the predictability of the volatility of the final product. The Grabner unit is a substantial advance in this respect. The Grabner unit (manufactured by Grabner Instruments) is a measuring device capable of providing Reid vapor pressure and vapor to liquid ratio data for a gasoline sample typically within 6-11 minutes of introducing the sample to the unit. The Distillation Process Analyzer (DPA) is another advance. The DPA (manufactured by Bartec) is a measuring device capable of provided a distillation temperature for a gasoline sample, typically within about 45 minutes of introducing the sample to the unit.
U.S. Pat. Nos. 7,032,629 and 6,679,302, PCT Patent Application No. WO 2007/124058, and U.S. Patent Application No. 2006/0278304 relate to methods and systems for blending butane and gasoline that ensure that the blended gasoline meets certain vapor pressure requirements. These references do not teach how to blend gasoline with more than one volatility modifying agent, and do not teach how to blend butane with gasoline that will subsequently be blended with ethanol.
U.S. Pat. No. 6,328,772 relates to the blending of gasoline and ethanol. The reference does not teach how to blend gasoline with more than one volatility modifying agent, and does not teach how to blend gasoline with butane.
Unfortunately, systems and methods have not been developed for mixing butane, ethanol, and gasoline to produce a blended gasoline that meets precise limits of volatility.